Impressive progress has recently been made in the field of optical telecommunications. Systems are now being installed that permit transmission of data at a rate of many megabits/sec over distances of several kilometers between repeaters. However, since the economics of systems, such as, for instance, intercontinental submarine cable systems, are strongly affected by data rate and repeater spacing, work directed towards improvement in such system parameters continues.
High data rate fiber telecommunication systems typically are, and will likely continue to be, digital systems, and this application is concerned only with digital systems. Furthermore, this application is concerned only with digital fiber telecommunication systems using monomode fiber.
Although currently available fiber can transmit signals with relatively low loss and low dispersion, and although further improvement in these parameters can reasonably be anticipated, fiber telecommunication links require, and likely will continue to require, regeneration of the signal at so-called "repeaters" at points intermediate between the sending or input end and the receiving or output end of the fiber communication channel. "Input end" and "output end" refer, of course, to a single transmission, and can be reversed for a subsequent transmission.
Repeaters typically carry out two functions, namely, raising the power level of the signal pulse, and reshaping the pulse. In addition, repeaters frequently also retime the pulse. Raising of the power level is required due to the attenuation suffered by the signal in any real fiber. Reshaping is required because, due to dispersive effects of the fiber, pulses typically spread. And retiming is found to be often necessary to maintain proper pulse spacing.
Repeaters in fiber telecommunication systems typically comprise means for detecting the signal, e.g., a photodiode, means for operating on the output of the photo detector, e.g., amplifying and reshaping the electrical output signal of the detector, and a source for optical radiation, modulated typically by the amplified and reshaped output signal of the detector, as well as means for again coupling the output of the optical source into the fiber. Repeaters of the type described are not only being used now but are being considered also for future fiber telecommunication systems. See, for instance, P. E. Radley, and A. W. Horsley, Proceedings of the International Conference on Submarine Telecommunication Systems, London, February 1980, pp. 173-176.
Conventional repeaters are typically complex devices containing a significant number of components. For instance, a typical optical regenerator contains around 50 transistors (ibid., page 174). This "electronic" complexity, particularly in high bit-rate systems, as well as reliability problems encountered with laser sources, is making repeater costs a major cost item for the fiber telecommunication systems that are currently under consideration.
The conventional response to these facts has, inter alia, been an effort to improve fiber quality, with the results that now repeater spacing of about 50 km appear feasible. Nevertheless, difficulties associated with the use of repeaters are sufficiently severe to make consideration of alternative solutions important, and this application pertains to such an alternative solution. We will next discuss some fiber characteristics relevant to the invention.
Pulses of electromagnetic energy transmitted through optical fiber experience attenuation and dispersion, with the latter resulting in a broadening of the pulse in the time domain. If such broadening is sufficiently severe, adjacent pulses can overlap, resulting in loss of signal detectability. In monomode fiber, (i.e., fiber in which only the fundamental mode of the signal can propagate at the operating wavelength of the system) the two principal dispersion mechanisms are material dispersion and waveguide dispersion. A material of index of refraction n exhibits material dispersion at the wavelength .lambda. if d.sup.2 n/d.lambda..sup.2 .noteq.0 at that wavelength. Physically, this implies that the phase velocity of a plane wave travelling in such a medium varies nonlinearly with wavelength, and consequently a light pulse will broaden as it travels through such medium. Waveguide dispersion typically also is wavelength dependent. We will refer herein to the combined material and waveguide dispersion as "chromatic" dispersion. As an example, typical of magnitudes of chromatic dispersion effects in a typical monomode fiber, a 10 ps pulse of carrier wavelength 1.5 .mu.m doubles its width after about 650 meters.
If in a medium d.sup.2 n/d.lambda..sup.2 &gt;0 throughout a certain wavelength regime, then the medium is said to be normally dispersive in that regime. On the other hand, a wavelength regime throughout which d.sup.2 n/d.lambda..sup.2 &lt;0 constitutes a so-called anomalous dispersion regime. In silica, for instance, a regime of normal dispersion extends from short wavelengths to about 1.27 .mu.m, and an anomalous dispersion regime from about 1.27 .mu.m to longer wavelengths. Separating the two regimes is a wavelength at which d.sup.2 n/d.lambda..sup.2 =0 i.e., at which material dispersion is zero to first order. This wavelength depends on the composition of the medium. The wavelength at which chromatic dispersion vanishes to first order similarly is composition dependent and, in addition, depends on such fiber parameters as diameter and doping profile. It can, for instance, be as high as about 1.5 .mu.m in appropriately designed monomode silica-based fibers.
A natural choice of carrier wavelength in a high data rate fiber telecommunication system is the wavelength of first-order zero chromatic dispersion in the fiber. However, even at this wavelength, pulse spreading occurs due to higher order terms in the dispersion. See, for instance, F. P. Kapron, Electronics Letters, Vol. 13, pp. 96-97, (1977).
Recently, it has been proposed to use the nonlinear change of dielectric constant (Kerr effect) of a monomode fiber to compensate for the effect of chromatic dispersion, i.e., to utilize "solitons." For purposes of this application, we mean by "soliton" a pulse of electromagnetic radiation that propagates in monomode optical fiber with a characteristic constant shape.
A soliton pulse occurs when the broadening effect due to chromatic dispersion is balanced by a contraction due to the nonlinear dependence of the index of refraction on electric field. The existence of solitons in monomode fiber and the possibility of their stationary transmission was predicted by A. Hasegawa and F. Tappert, Applied Physics Letters, Vol. 23(3), pp. 142-144, (1973). That paper dealt with lossless monomode fibers, and taught the existence of a minimum pulse peak power, dependent, inter alia, on fiber parameters, pulse width and carrier wavelength, above which solitons can exist. These predictions of Hasegawa and Tappert have been verified, for instance, by demonstrating dispersionless transmission of a 7 ps pulse with a peak power of about 1 Watt at 1.45 .mu.m through monomode fiber for a distance of about 700 meters. See, L. F. Mollenauer et al, Physical Review Letters, Vol. 45(13), pp. 1095-1098, (1980). Mollenauer et al also verified the prediction by Hasegawa and Tappert that soliton pulses of peak power in excess of the so-called "balanced" peak power P.sub.o undergo pulse narrowing.
Recently, A. Hasegawa and Y. Kodama have proposed the use of soliton pulses in high data rate monomode fiber telecommunication systems. See Preceedings of the IEEE, Vol. 69(9), September 1981, pp. 1145-1150. That paper contained an extensive discussion of the properties of solitons in ideal optical fiber, of the effects of higher order dispersion and of loss on solitons, as well as design examples and criteria, and is incorporated herein by reference.
Utilization of the Kerr effect to achieve pulse self-confinement in multimode fibers has also been proposed recently. U.S. patent application, Ser. No. 230,322, now U.S. Pat. No. 4,368,543 filed Feb. 2, 1980 by A. Hasegawa, entitled "Multimode Fiber Lightwave Communication System Utilizing Self-Confinement." Both the proposed monomode and the proposed multimode telecommunication systems utilize the self-confinement effect to achieve high rates of data transmission. They do, however, not address the question of pulse regeneration, and the difficulties inherent in conventional approaches of regeneration that were alluded to above.